Species Networks

Author

cslovas

Species Grouping

To correlate species distribution changes with changes in license holdings in Maine, license types have been grouped by target species as described by Maine State Legislation.

License Code Description Target Species
al Aquaculture License: Established in 2017 Aquaculture
cfc Commercial fishing crew Finfish
cfs Commercial fishing single Finfish
cpc Commercial pelagic and anadromous crew Pelagic/Anadromous
cps Commercial pelagic and anadromous single Pelagic/Anadromous
cs Commercial Shellfish Shellfish
csc Commercial shrimp crew Shrimp
cso Commercial shellfish + 70 Shrimp
css Commerical Shrimp Single Shrimp
csu Commercial Shellfish Under 18 Shellfish
csw Commercial Shrimp Single Shrimp
csws Commercial Shrimp Crew Shrimp
e0 Elver Dip Net Elver
e0c Elver Dip Net Crew Elver
e1 Elver 1 Fyke Net Elver
e1c Elver 1 Fyke Net Crew Elver
e2 Elver 2 Fyke Net Elver
e29 Elver Non Resident Elver
e2c Elver 2 Fyke Net with Crew Elver
e6 Elver Dip Net 1 Fyke Net Elver
e6c Elver Dip Net 1 Fyke Net Crew Elver
eel Eel license - changed to Eel Pot Eel
eex Elver Exporter Elver
elv Elver Elver
ep eel pot/hoop net Eel
fhc federal halibut crew Halibut
gc Green crab Green crab
gcn green crab non resident Green crab
la Lobster Crab Apprentice Lobster
lao Lobster Crab Apprentice + 70 Lobster
lau lobster crab apprentace under 18 Lobster
lc1 Lobster crab class I Lobster
lc2 Lobster crab class II Lobster
lc2o Lobster crab class II + 70 Lobster
lc3 Lobster crab class III Lobster
lc3o Lobster crab class III + 70 Lobster
lco Lobster Crab + 70 Lobster
lcs Lobster Crab Student - policy change in 2016 Lobster
lcu Lobster Crab Under 18 Lobster
lnc Lobster crab non commerical Lobster
mb Mussel Boat Mussel
md Mussel Dragger Mussel
menc menhaden commercial Menhaden
mennr menhaden commercial non-res Menhaden
menr menhaden non commercial Menhaden
mh Mussel Hand Mussel
mwd Marine Worm Digging Marine worm
ncf commercial fishing non resident Finfish
ncp non-resident pelagic and anadromous Pelagic/Anadromous
ncs Scallop non commericial Scallop
nfhc non federal halibut crew Halibut
nfhs non federal halibut single Halibut
nla Non-res lobster apprentice Lobster
nlc1 Non-res lobster class 1 Lobster
nrlp Non-res lob crab landing Lobster
nrs Seaweed non resident Seaweed
nrss seaweed non resident supp Seaweed
ntp non-resident tuna permit Tuna
qm Quahog (Mahogany) Quahog clam
sb Scallop boat - from 1990 to 1993 - then went to SB Scallop
scd Sea Cucumber Drag Sea cucumber
sd Scallop Dragger Scallop
sdi Scallop diver Scallop
sh Scallop Hand - now SDI changed in 1994 Scallop
spt spat Spat
sub Sea Urchin Dragger Sea urchin
suh Sea urchin diver Sea urchin
sur Sea Urchin Raking Sea urchin
surf Surf Clam Boat Surf clam
sw Seaweed Seaweed
swbn Nonresident Seaweed Buyer Seaweed
sws seaweed supplemental Seaweed

License Portfolio by Species

Using Theresa Burnham’s code for building the individual license portfolio, license types were grouped by target species. A binary matrix was created, whereas 1 denotes an individual license holding for a corresponding year and 0 denotes no license held. With this co-occurrence matrix, we can quantify which license types were held together over the past 30 years and how those holdings have changed.

Species Proportions

Species Matrix

Species Aquaculture Eel Elver Finfish Green crab Halibut Lobster Marine worm Menhaden Mussel Pelagic/Anadromous Quahog clam Scallop Sea cucumber Sea urchin Seaweed Shellfish Shrimp Spat Surf clam Tuna
Aquaculture 0 3 5 33 43 0 293 13 29 23 17 1 34 0 9 26 149 15 40 0 0
Eel 3 0 173 270 74 0 260 60 11 55 38 35 72 0 58 88 140 45 0 13 0
Elver 5 173 0 1793 121 1 4646 1067 65 147 95 36 1261 0 1373 194 3206 326 0 0 0
Finfish 33 270 1793 0 463 17 29510 1758 10145 670 5480975 652 10182 54 6093 1206 6205 4968 21 8 2
Green crab 43 74 121 463 0 0 754 254 39 64 80 42 258 5 115 169 656 134 5 6 0
Halibut 0 0 1 17 0 0 21 0 2 0 5 0 3 0 0 0 3 0 0 0 0
Lobster 293 260 4646 29510 754 21 0 3933 2262 982 1732 994 21390 72 12567 1555 16165 6715 66 42 1
Marine worm 13 60 1067 1758 254 0 3933 0 12 93 33 58 976 0 1175 589 5552 155 0 2 0
Menhaden 29 11 65 10145 39 2 2262 12 0 15 5310 24 291 2 53 32 187 4 18 4 0
Mussel 23 55 147 670 64 0 982 93 15 0 44 189 916 18 451 287 465 154 3 14 0
Pelagic/Anadromous 17 38 95 5480975 80 5 1732 33 5310 44 0 45 567 15 133 88 211 394 11 7 0
Quahog clam 1 35 36 652 42 0 994 58 24 189 45 0 1186 8 619 52 257 184 2 15 0
Scallop 34 72 1261 10182 258 3 21390 976 291 916 567 1186 0 114 9837 621 4263 2549 12 17 0
Sea cucumber 0 0 0 54 5 0 72 0 2 18 15 8 114 0 54 0 3 29 0 0 0
Sea urchin 9 58 1373 6093 115 0 12567 1175 53 451 133 619 9837 54 0 606 4156 769 2 0 0
Seaweed 26 88 194 1206 169 0 1555 589 32 287 88 52 621 0 606 0 773 163 4 13 0
Shellfish 149 140 3206 6205 656 3 16165 5552 187 465 211 257 4263 3 4156 773 0 520 9 9 0
Shrimp 15 45 326 4968 134 0 6715 155 4 154 394 184 2549 29 769 163 520 0 0 6 0
Spat 40 0 0 21 5 0 66 0 18 3 11 2 12 0 2 4 9 0 0 0 0
Surf clam 0 13 0 8 6 0 42 2 4 14 7 15 17 0 0 13 9 6 0 0 0
Tuna 0 0 0 2 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Annual Species Metrics

Degree and betweenness are both ways of measuring centrality.

“The degree of a node is the number of other nodes that single node is connected to.

Important nodes tend to have more connections to other nodes. Highly connected nodes are interpreted to have high degree centrality.”

“[Betweenness] ranks the nodes based on the flow of connections through the network. Importance is demonstrated through high frequency of connection with multiple other nodes. Nodes with high levels of betweenness tend to serve as a bridge for multiple sets of other important nodes.” Source

Lobster is the most abundant license type held in Maine, its high degree of centrality with relatively low betweenness suggests that while lobster has a lot of connections to other license types, those license types that co-occur with lobster don’t co-occur with many other license types.

Eel and quahog have highly variable betweenness with relatively low degree of centrality, suggesting that these license may be held concurrently with more central license types to supplement harvest.

Finfish also has a high degree of centrality and low betweeness.

Annual Network Plots

Similar to plotting the license holdings as a network over time, we plot the target species pertaining to these permits over time. This makes for a more easily interpretable graph, as well as increased potential for correlating license changes to species distribution changes

The thickness of lines across the network reflect the frequency of co-occurrence. Strong connections between all lobster license types and commercial finfish appear consistently over the 33 year data set. However, since the mid-90’s there have been more and more connections occurring at lesser frequencies. The connection of finfish to lobster stays relatively consistent, suggesting that harvesters are diversifying license holdings rather than exiting entirely.

Limited Entry Fisheries

According to Maine State Legislation, there are 6 limited entry species including lobster, which is limited by harvest zone. Most species became limited entry in response to a fishery collapse, while some aim to avoid collapse. The most recent limited entry fishery is menhaden, which was passed by the legislature in 2023.